The present invention relates to an image decoding apparatus and decoding method and, more particularly, to an image decoding apparatus and decoding method suitable for decoding image data encoded by the H.264 format.
The international standardized encoding methods such as MPEG-4 are known as motion image encoding techniques, and ITU-Rec. H.264 Advanced Video Coding ISO/ITE International Standard 14496-10 (MPEG-4 AVC) abbreviated as H.264 is known as a new encoding method.
A variable-length code decoding device decodes a prefix level_prefix and suffix level_suffix by the following procedure. First, a table selection controller outputs a syntax selection signal so as to select a TrailingOnes syntax from the syntaxes level_prefix, level_suffix, and TrailingOnes. Accordingly, the variable-length code decoding device outputs the TrailingOnes syntax.
Then, the table selection controller outputs a syntax selection signal so as to select the prefix level_prefix syntax. The variable-length code decoding device outputs the prefix level_prefix by using a predetermined table.
A suffix level_suffix decoding determination device receives the decoded prefix level_prefix, and determines the presence/absence of the suffix level_suffix in accordance with the manual of H.264. The suffix level_suffix decoding determination device outputs the result to the table selection controller.
On the basis of this result, if the suffix level_suffix exists, the table selection controller outputs a syntax selection signal so as to select the suffix level_suffix. On the basis of a suffix length suffixLength output from a device for updating the suffix length suffixLength, the variable-length code decoding device decodes the suffix length suffixLength by using a predetermined table.
On the other hand, a level formation device calculates a level by using the syntaxes (TrailingOnes, prefix level_prefix, and suffix level_suffix) output from the variable-length code decoding device, and outputs the result to a suffix length (suffixLength) updating device.
The suffix length (suffixLength) updating device updates the suffix length suffixLength by using the value of the level output from the level formation device, and prepares for decoding the next suffix level_suffix.
When decoding the prefix level_prefix and suffix level_suffix by using the arrangement as described above, however, if the suffix level_suffix exists, the prefix level_prefix is first decoded, and then the suffix level_suffix is decoded by using the decoding result. That is, two cycles are required. This means that decoding of one residual data requires two cycles.
The residual data occupies most of the encoded image data. If the number of decoding cycles of this residual data increases, therefore, the operational frequency necessary for the required decoding performance of the system must be raised. This unavoidably increases the power consumption.
A reference disclosing the conventional image decoding technique is as follows.
Japanese Patent Laid-Open No. 2006-157157